1. Field of the Invention
The present invention is broadly concerned with improved mud additive compositions particularly formulated for the treatment of oil wells characterized by the presence of downhole iron, in order to minimize hole enlargement when salt beds are encountered during the drilling process, and to reduce the need for fresh water in the control of salt deposition within the well. More particularly, the invention pertains to an aqueous composition including a first ingredient such as sodium or potassium ferrocyanide and mixtures thereof and a second ingredient such as a phosphonic acid, an alkali or alkaline earth metal phosphonic acid salt, and mixtures thereof. The composition is added to recirculating drilling mud and lessens salt erosion even in the case of wells having high concentrations of iron therein. The composition may be added to circulation water to enhance solubilization of salt within the well.
2. Description of the Prior Art
During the course of oil well drilling operations, it sometimes occurs that a salt bed is encountered. When this happens, typical clay-based aqueous drilling mud can quickly erode the salt formation by dissolving salt, thereby creating a washed out annular zone. When this occurs, additional cement is needed to fill the hollowed out zone, thereby increasing drilling costs. The drill string may also become stuck as fluid velocity falls below cutting particle slip velocity in the widened zone, thereby causing particles to settle and bind the drill string. Furthermore, the hole enlargement greatly complicates formation evaluation efforts.
In response to these problems, it is conventional practice to add salt or saturated brine to the drilling mud through a hopper mixing system or fluid jetting system at the mud pit. In this way, annular erosion is lessened. However, this expedient is not entirely satisfactory, owing to the fact that downhole conditions of increasing temperature with depth enable brines that are salt-saturated at surface temperatures to dissolve additional salt in the well bore. As the resulting super-saturated brines are circulated to the surface and cooled, salt precipitates from the aqueous system. This precipitation can create a disposal problem in the pits and enables the brine to dissolve additional well bore salt during the next circulation.
Drilling mud additives have been commercialized in the past which serve to prevent dilution of the brine fraction of the recirculating drilling mud. Sodium ferrocyanide is a very effective and relatively inexpensive treating agent, and is therefore preferred. However, sodium ferrocyanide has a significant drawback in that if the well and its products contain iron, the effectiveness of the treatment is drastically reduced, and a blue, scale-like amorphous substance is deposited on downhole equipment. This leads to downtime and loss of production, and even a need to replace downhole equipment.
To overcome this problem a scale inhibiting agent has been used along with sodium ferrocyanide. These scale inhibiting agents are compounds such as the trisodium salt of nitrilotriacetic acid (NTA) and alkali metal citrates as described in U.S. Pat. No. 5,344,818. These scale inhibiting agents generally are capable of sequestering about 250-300 ppm of dissolved iron. This may seem adequate considering that a typical well characterized by the presence of downhole iron will contain a dissolved iron concentration of 50-100 ppm, however, as the age of an oil field increases incidences of iron-containing deposits, like rust, on pipe casings and other downhole equipment also tends to increase. During treatment, the scale inhibiting solution comes into contact with these pipe casings and pieces of equipment, thereby dissolving some of the built-up, iron-containing deposits. The dissolution of iron may become so great that by the time the scale inhibiting solution reaches the well bottom, the solution is already saturated with iron and unable to sequester the downhole iron.
There is accordingly a real and unsatisfied need in the art for an improved composition making use of an inexpensive ferrocyanide for brine control during drilling and well operations, while at the same time having the capacity to continue to sequester downhole iron even though being injected through aging well equipment containing built up iron deposits.
The present invention overcomes the problems outlined above, and provides a downhole well treating composition comprising (and preferably consisting of) a first ingredient selected from the group consisting of sodium and potassium ferrocyanide and mixtures thereof and a second ingredient selected from the group consisting of a phosphonic acid, alkali and alkaline earth metal phosphonic acid salts and mixtures thereof. Such a composition is supplied in an aqueous medium at the well site. At this point the liquid composition may be introduced into the well, typically by addition thereof to the recirculating drilling mud and/or to fresh water. It has been found that not only will this composition effectively control the salt erosion problem, but also maintains the capacity to sequester downhole iron and thereby eliminate the characteristic blue scale formation encountered in wells having significant downhole iron contents even though injected through aging well equipment containing built-up iron deposits.
Preferred particulate, dilutable downhole well treatment compositions in accordance with the invention include from about 20-95% by weight of a first ingredient selected from the group consisting of sodium and potassium ferrocyanide and mixtures thereof, with the sodium salt being the most commonly employed. The compositions also include from about 5-50% by weight of a second ingredient taken from the group consisting of a phosphonic acid, alkali and alkaline earth metal phosphonic acid salts and mixtures thereof, Dequest 2066A being preferred. All weight percentages are based on the weight of the entire composition being 100%. These levels are even more preferably from about 55-75% by weight for the first ingredient, and correspondingly from about 25-45% by weight for the second ingredient. The single most preferred levels of use are about 65% by weight for the first ingredient, and about 35% by weight for the second ingredient.
In actual practice, the particulate composition described above is dispersed in an aqueous medium. A preferred embodiment of the solution should contain between 1-12% by weight of sodium ferrocyanide and about 4-50% by weight of Dequest 2066A, more preferably about 25%. All weight percentages are based on the total weight of the solution being 100%. Soda ash is then added to the solution in sufficient amount to adjust the pH of the solution to about 7.
The aqueous medium may simply comprise plain water or brine, however different compounds may be added to enhance specific characteristics of the aqueous medium. For example, the aqueous medium may comprise an amount of a glycol, such as ethylene glycol in order to keep the solution from freezing if stored or used in cold environments. Those skilled in the art can readily discern the appropriate amount of ethylene glycol to include, depending upon the temperature extremes involved in the particular operation; preferably the solution will contain between 10-40% by weight ethylene glycol. Additionally, preferred embodiments of the invention contain from about 1-10% by weight sodium citrate and from about 1-10% by weight sodium erythorbate. All weight percentages are based on the total weight of the solution being 100%. The pH of the final product should be from about 6-14 with an optimum pH of about 8.5. The final product will be able to sequester at least about 10,000 ppm and up to about 50,000 ppm of iron without precipitation of iron compounds.
The aqueous treating compositions of the invention are introduced into an oil well in order to control the salt erosion problem, even in the case of wells having significant iron contents. While such introduction may be effected in a number of ways, generally speaking it is preferred to add the liquid compositions to the recirculating drilling mud. Typically, this is done by addition of the agents at the mud pit where jets can be used to stir the supplemented mud prior to recirculation downhole. The compositions should be added to the drilling mud at a level of from about 30-250 ppm, and more preferably at a level of from about 50-170 ppm of the drilling mud.
In addition, the compositions hereof may be added to fresh circulation water at the ppm levels indicted above, to serve as a medium for dissolving deposited salt on downhole well equipment. Use of the composition hereof reduces the amount of fresh water needed for this purpose.
The aqueous treating composition is not limited to use strictly in oil wells. The composition may be used to treat other systems, such as pipelines, which are characterized by the presence of iron-containing deposits.
Sodium ferrocyanide is the most preferred brine-control agent for use in compositions of the invention. Sodium ferrocyanide is available from JACAM Chemicals, L.L.C. of Sterling, Kans., under the designation of xe2x80x9cDeSaltxe2x80x9d or xe2x80x9cSalt Inhibitorxe2x80x9d.
It has been found that the addition of sodium or potassium ferrocyanide to brine increases the concentration of sodium chloride therein. When crystallization of the sodium chloride takes place, the resultant salt crystals become more pyramidal (dendritic salt) instead of cubical. This increased salt concentration in the brine renders it particularly useful in the context of oil well treatments.
The most preferred second ingredient is known by the trade name xe2x80x9cDequest 2066Axe2x80x9d and is the trisodium salt of the following phosphonic acid: 
where x is equal to 2. Dequest 2066A is available from Solutia, Inc. of St. Louis, Mo. A product information sheet distributed by Solutia describing the Dequest 2066A product is incorporated herein. In addition to Dequest 2066A, the above phosphonic acid, alkali or alkaline earth metal salts or mixtures thereof where x has a value of from 2 to 6 may also be employed.